The present invention relates to magnetic particles and a magnetic carrier for an electrophotographic developer, comprising the magnetic particles. More particularly, the present invention relates to magnetic particles for use as an electrophotographic magnetic carrier in an electrophotographic developer, an electrophotographic magnetic carrier for an electrophotographic developer which have an excellent durability and a stable charging property, using the magnetic particles, and an electrophotographic developer using the electrophotographic magnetic carrier.
In electrophotographic developing methods, a photosensitive member composed of a photoconductive material such as selenium, OPC (organic semiconductor), a--Si (amorphous silicon) or the like has been used to form an electrostatic latent image thereon by various means. Then, by using a magnetic brush method or the like, a toner having a polarity reverse to that of the latent image is attached thereon to form the latent image by the electrostatic force.
As is well known in the art, in the above developing methods, there have been used support particles called a magnetic carrier. The magnetic carrier acts for imparting an appropriate positive or negative electrical quantity to the toner by frictional electrification, and transferring the toner into a developing zone near the surface of the photosensitive member by a developing sleeve in which magnets are accommodated, using the magnetic force thereof.
In recent years, the electrophotographic developing method has been widely applied to copying machines or printers. In these apparatuses, it has been demanded to meet various requirements including not only reproduction of thin lines, small characters, photographs, color originals or the like, but also a high image quality, a high image grade, a high copying or printing speed, a continuous image formation or the like. The requirements for these properties have been estimated to become increased more and more in future.
In order to satisfy not only the applicability to various objectives but also the high image quality and the high image grade, the reduction in a particle size of the toner particles and the magnetic carrier particles, has been studied. In particular, it has been strongly demanded to provide magnetic carrier particles having an average particle size as small as 10 to 50 .mu.m.
On the other hand, in order to satisfy the high copying or printing speed and the continuous image formation, it has been demanded to enhance the durability of these particles as developer. In the case of the magnetic carrier, there has been proposed such a method which comprises iron particles obtained by a mechanical pulverization method, an electrolytic method, a reduction method, a heat-decomposition method, a sintering method or the like; granulating and then heat-sintering various ferrite fine particles or magnetite fine particles to form granulated sintered particles; dispersing magnetic particles or magnetic particle and non-magnetic particles in a binder resin to form composite particles (hereinafter referred to merely as "magnetic core particles"); and then coating the surfaces of the obtained magnetic core particles with various resins. The above magnetic carrier has been already put into practice. (Japanese Patent Publication (KOKOKU) No. 2-3181, and Japanese Patent Application Laid-open (KOKAI) Nos. 62-66269 and 3-242657, etc.)
There is no end of a demand for the enhancement in properties of the electrophotographic developers. In order to continuously obtain a clear image, it is desired that the charge amount of the magnetic carrier is kept unchanged and stable even after the magnetic carrier is used for a long period of time. Specifically, when the magnetic carrier is used for a long period of time, there arises such a problem that the coating resin layer is peeled off from the surfaces of the magnetic core particles, so that the charging property of the magnetic carrier is deteriorated, whereby the magnetic carrier cannot impart an appropriate charge to the toner. Therefore, it has been demanded that the coating resin layer can be prevented from being peeled off from the surfaces of the magnetic core particles in order to enhance the durability of the magnetic carrier, thereby allowing the magnetic carrier to show a more stable charging property.
Hitherto, in order to enhance the durability of the magnetic carrier, there have been proposed a magnetic carrier obtained by coating the surfaces of magnetic core particles with a silicone resin containing a silane-based coupling agent (Japanese Patent Application Laid-Open (KOKAI) No. 5-107819(1993), etc.); or the like.
At the present time, it has been strongly required to provide an electrophotographic magnetic carrier having an excellent durability and a stable charging property. However, such a magnetic carrier has not been obtained yet.
That is, in the above-described known magnetic carriers, since the silane-based coupling agent contained in the silicone resin is bonded to hydrophilic groups such as hydroxy groups which are present on each surface of the magnetic core particles, so that the silicone resin composition-coating layer is unlikely to be peeled off as compared to the case where a silicone resin-coating layer which contains no other component is directly formed onto each surface of the magnetic core particles. However, as described in Comparative Examples hereinafter, when such known magnetic carriers are repeatedly used for a long period of time, the silicone resin composition-coating layer finally starts to be peeled off. Thus, the known magnetic carriers are still unsatisfactory in durability. Further, the charging property of the known magnetic carriers tends to be fluctuated.
As a result of the present inventors' earnest studies, it has been found that by forming a resin composition-coating layer comprising a metal curing agent, a silane-based coupling agent oligomer and a silicone resin, on the surface of the magnetic core particle, the obtained magnetic particles are useful as a magnetic carrier for an electrophotographic developer. The present invention has been attained on the basis of this finding.